Wide band transponder

ABSTRACT

A simple low-power transponder adapted for rescue purposes employs high frequency diode oscillator-detector devices each serving the dual role of receiver and transmitter in response to illumination by energy transmitted by a conventional search radar system.

United States Patent 1191 [111 3,728,630

Strenglein 5] Apr. 17, 1973 [54] WIDE BAND TRANSPONDER 3,383,682 5/1968Stephens, Jr. ..343/8 X 2,561,421 7/1951 Sealeetal [75] Inventor: HarryF. Strenglem, Clearwater, 3 373,425 3/1968 Barischoff .::..343/6.8 R3,389,391 6/1968 Keeler et a1 ..343/6.8 R [73] Assignee: Sperry RandCorporation, New

York NA! Primary Examiner-Ma1co1m F. Hubler Att0rneyS. C. Yeaton [22]Filed: Feb. 16, 11971 21 App1.No.: 115,508

[57] ABSTRACT A simple low-power transponder adapted for rescue [58]Fieid 343/6 R 6 8 LC purposes employs high frequency diodeoscillator-de- 343/18 B; 325/19 8 tector devlees eech serving the dua1role of rece1ver and transmltter 1n response to lllummatlon by energy[56] References Cited transmitted by a conventional search radar system.

UNITED STATES PATENTS 11 Claims, 1 Drawing Figure 2,548,813 4/1951Pericins et a1. ..343/6.8 R

OSCILLATOR g 160 V 5a 60 3 la -i A AMFLlFlER a OSCILLAIUR ILTECTOR x2:

WUENTEDAFR1 7 OSC l L ATOR D ETECTOR PULSE AMPLIFIER GENERATOR SZ/9 fl33 S 1+ 8 I AS I 7 SOURCE .I 150- D 6a 5 60 S Z0 14 .AMPL IER OSCILLATORDETECTOR x2: 32/90 IN VENTOR HARRY F. STRENGLE/N ATTORNEY WIDE BANDTRANSPONDER BACKGROUND OF THE INVENTION neither receiver nor transmittertuning and adapted, for

example, for use in distress, emergency, and rescue situations.

2. Description of the Prior Art A considerable variety of beacon orradar transponder devices is known in the prior art of the kind whichrespond by transmitting a high frequency pulse lying in the receptivityband of a radar receiver when interrogated by a pulse transmitted by thecooperating transmitter of a radar system. Various prior requirements ofoperation have generally resulted in the production of radartransponders which are complex and have the inherent large size, weight,and initial and maintenance cost characteristics which are often inherent in complex designs. Such devices normally have specialcapabilities for generating pulsed high frequency transmissions that canbe received by conventional radar receivers. Some utilize a harmonicgenerator cooperating with a receiver tuned to a harmonic of theexpected radar carrier frequency. Others employ various types of mixeror frequency converter systems for regeneration of high frequencysignals from the received radar signal. Still others use complexautomatic frequency control systems for tuning elements of thetransponder so that it transmits at the same carrier frequency as theinterrogating radar transmitter. All have one or another featureunsuitable to use in emergency or rescue operations. These featuresinclude high cost, a complexity incompatible to the applications, highcurrent drain, or a need for operation with a radar receiver differentfrom that commonly used in search radar systems.

SUMMARY OF THE INVENTION The present invention relates to high frequencytransponder or radar beacon devices for greatly enhancing the echoreturns of search radar systems in search and rescue operations. Whilethe novel radio transponder has other applications, one which may beparticularly mentioned is its use when airborne or surface radar systemsare employed in an attempt to locate small craft in distress. Forexample, most small marine vessels, especially when constructed ofwooden or fiber glass materials, are not particularly conspicuoustargets readily identified in a search radar indicator against strongsea clutter. The problems connected with locating life boats or raftsand swimmers are increasingly difficult of solution. The inventionemploys low-power transponding elements in the form of high frequencydiode oscillator-detector devices each serving a dual role as receiverand transmitter when interrogated by pulsed high frequency carrierenergy transmitted by a conventional search radar system. The need forreceiver tuning is eliminated by utilizing a broad band detector; theneed for transmitter tuning is removed by transmitting a pulse having avery broad frequency spectrum. In the particular embodiment of theinvention, spreading of the spectrum is accomplished by transmitting ashort duration pulse which inherently has a broad spectrum. Theoscillator-detector unit which is capable of transmitting very shortpulses involves a resonant circuit having a low quality factor Q; thisfeature enables the device to work effectively also as a broad banddetector. The invention provides means for greatly increasing thechances of location of small craft in adverse conditions. I

BRIEF DESCRIPTION OF THE DRAWINGS The sole FIGURE is an electricalwiring diagram illustrating the components of the invention and theirinterconnections.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the sole FIGURE, the wideband transponder is indicated for purposes of illustration as having twosimilar operating channels. It is to be understood that the inventionmay also be practiced using a plurality of channels greater than two,and that under certain practical circumstances, a single channel will beuseful. Accordingly, a discussion of the structure and operation of asingle channel of the transponder will first be presented. Therepresentative channel to be discussed may include circuit elements 1through 17.

Antenna 1 is preferably a conventional high frequency or microwaveantenna of the omnidirectional type having a moderate degree ofdirectivity in elevation. For example, when the invention is used as anemergency device when at sea, a rescue vessel having a radar unitsuitable for excitation of the novel transponder system will generallyhave any random bearing with respect to a small vessel, raft, or othervehicle on which the emergency transponder may be located. An antennacapable of good response at substantially all azimuth angles istherefore desirable. Accordingly, a conventional monopole antenna may beemployed or one may use other omnidirectional antennas such as the stubantenna, the top loaded stub antenna, the disc antenna, or the flushdisc antenna Any pulsed high frequency carrier energy collected byantenna 1 is coupled into the quiescent single-port oscillator-detector2, causing amplified oscillations to prevail within oscillator-detector2 for a time period generally related to the amplitude and time durationof the received carrier pulse. It is to be understood that single-portoscillator-detector 2 may be any of several conventional knownoscillator-detector devices or circuits, including an appropriatelyadjusted avalanche transit time high frequency diode oscillator of thetype which is well known in the art to act as a detector when biased ata near oscillating condition. The single-port device 2 operates in theinvention both as a detector and as an oscillator. For example, when theoscillators diode is biased near the point at which oscillations wouldcommence, the added voltage across the oscillator diode, when a highfrequency carrier is injected from antenna 1, produces growingoscillations because of conventional gain mechanisms inherent in theoscillator. These oscillations quickly decay when the input carriersignal ends. They can be measured or detected by observing the level ofthe diode bias current in the oscillating and non-oscillatingconditions. Likewise, when the oscillator diode is biased still near thesame point at which oscillations would commence, a slight elevation ofthe bias voltage will cause the circuit to break into high frequencyoscillations. While conventional high frequency avalanche diodeoscillators represent one form of negative resistance device suitablefor dual mode operation as oscillators and detectors, other suitabledevices are present in the prior art, including Gunn and tunnel diodeoscillators and other similar amplifier or oscillator devices.

Since the particular form of the oscillator-detector device is notnecessarily a part of the present invention, a discussion of details ofits structure is not required for an understanding of the invention.However, singletuned diode oscillator or amplifier devices suitable foruse in the invention are described liberally in the literature and inthe C. T. Rucker, J. W. Amos U.S. patent application Ser. No. 853,725,filed Aug. 28, 1969 for a Microwave Negative Resistance Transducerissued Sept. l4, l97l as U.S. Pat. 3,605,034 and assigned to the SperryRand Corporation.

As seen in the sole FIGURE, diode oscillator detector 2 is supplied witha suitable diode biasing voltage through resistor 4 from a conventionalbias signal source 3. By virtue of the presence of resistor 4, the biaslevel supplied to the diode of single-port oscillatordetector 2 is verynearly sufficient to cause oscillations to grow withinoscillator-detector 2. The arrangement is such that the presence ofreceived high frequency carrier energy collected by antenna 1 initiatessuch oscillations with the further consequence that there is an abruptincrease in current supplied by bias source 3 to the oscillator-detector2 when oscillations begin. It is noted that the bias current pathincludes ground, bias source 3, resistor 4, the diode of device 2, andthe return lead to ground.

A voltage corresponding to the instantaneous bias current appears acrossresistor 4 and any change in that bias current is coupled by capacitor 5through current limiting resistor 6 to pulse amplifier 7. Capacitor 5also prevents bias current flow into resistor 6. The junction betweenresistor 6 and pulse amplifier 7 is coupled to ground through a dualdiode clipper circuit composed of oppositely poled diodes 8 and 9.Amplifier 7 thus produces a pulse output substantially when carrierenergy is received by antenna 1 and the pulse output is supplied throughdiode 10 to a conventional pulse generator circuit 11. Pulse generator11 may be a conventional single shot multibrator or other pulsegenerator which creates a pulse of duration, for example, of the orderof 10 to 40 nano-seconds at sufficient amplitude to produce high poweroscillations in oscillatordetector 2. Any pulse output developed bypulse generator 11 is coupled back by electrical conductor 13 tojunction 14. Junction 14 cooperates with a series circuit connectedbetweenjunction l4 and junction 17, which latter junction lies betweencoupling capacitor 5 and the current limiting resistor 6. The seriescircuit between junctions l4 and 17 consists of a diode l5 poled toconduct current flowing toward terminal 17 only, diode being placed inseries with a resistor 16.

In operation, an input carrier pulse transmitted, for example, by asearch radar system in the operating vicinity of antenna 1 causes thequiescent single-port oscillator-detector device 2 to break intooscillation. Such operation causes device 2 to draw increased currentthrough resistor 4 from bias source 3, otherwise such oscillations couldnot build up in device 2. Since the bias current level is reflected in avoltage appearing across resistor 4, capacitor 5 couples an impulsecoinciding with the front edge or start of oscillations in device 2through capacitor 5, resistor 6, and pulse amplifier 7 to pulsegenerator 1 1. The diode clipper circuit 8, 9 confines the totalexcursion of the impulse with respect to ground potential in theconventional manner so as to protect amplifier 7 from over load. Sincepulse generator 11 may also receive signals from other channel sources,diode 10 is provided to permit current flow from amplifier 7 only in thedirection of generator 11. Pulse generator 11 produces a pulsepreferably short enough to produce the desired radio spectrum width.Pulse generator 11, like many other conventional pulse generators, has afinite recovery time, preventing its regenerative operation. Therefore,it must await a newly received input pulse traversing capacitor 5 andpulse amplifier 7 before it is in condition to generate a succeedingoutput pulse.

The standardized pulse formed by pulse generator 11 is fed by electricalconductor 13 to junction 14, thus passing also to junction 17. Diode l5permits flow of the pulse signal only in the direction from junction 14toward junction 17, where the pulse is coupled by capacitor 5 tooscillator-detector device 2. The polarity of the pulse thus supplied issuch as to aid the bias source 3 in causing oscillator-detector 2 againto break into high frequency oscillation, which oscillations arethereupon radiated by antenna 1 to be received by the open receivers ofradar apparatus in the operating vicinity of the transponder and thevehicle upon which it is mounted. In conventional radar apparatus, therange and bearing of the vehicle carrying the transponder of the presentinvention is readily presented in a conventional manner on a type P orplan position cathode ray indicator.

Where individual transponder power output and band width characteristicsso determine, multiple channels may be employed to enhance operation ofthe invention. The apparatus of the sole FIGURE illustrates the use oftwo similar channels, though three or more may be employed. In thesecond channel of the sole FIGURE, for instance, a secondomnidirectional antenna la is employed, though its function may becombined with that of antenna 1. However, a second singleportoscillator-detector device 2a is used, designed to cover a frequencyrange usually adjacent that of oscillater-detector device 2. Device 2ais supplied with an operating bias potential via resistor 4a from thecommon bias source 3. Again, by virtue of resistor 4a, the bias levelsupplied to the diode of a single port oscillator-detector 2a is verynearly sufficient to cause oscillation within oscillator-detector 2a.The presence of received high frequency carrier energy in antenna lainitiates such oscillation with the consequence that there is an abruptincrease in current supplied by bias source 3 to the oscillator-detector2a.

A voltage corresponding to the instantaneous bias current appears acrossresistor 4a and any change in that bias current is coupled by capacitor5a through limiting resistor 6a to amplifier 7a. The junction betweenresistor 6a and amplifier 7a is coupled to ground through a conventionaldual diode (8a, 9a)

clipper circuit analogous to the clipper circuit including diodes 8, 9.Amplifier 7a thus produces an impulse output when carrier energy isreceived by antenna la and that output is supplied through diode 10a tothe pulse generator 11. As previously noted, any pulse output developedby pulse generator 11 is coupled back by electrical conductor 13 tojunction 14. In the second channel, junction 14 cooperates with a secondseries circuit now connected between junction 14 and junction 17a, whichlatter is the junction between coupling capacitor 5a and the currentlimiting resistor 6a. The series circuit between junctions l4 and 17aconsists of diode 15a poled to conduct current flowing toward terminal17a only, diode 17a being placed in series with resistor 16a.

Operation of the first and second described channels is similar. Anincoming high frequency signal falling in the pass band of theoscillator-detector 2 causes the latter to oscillate and to provide adetected trigger pulse to the pulse generator 11. The latter is fed backvia electrical conductor 13 and along the signal paths which branch atjunction 14 respectively through coupling condensers 5, 5a, causing bothof the oscillator-detectors 2, 2a to oscillate and to radiate highfrequency energy from antennas 1, la. In a similar fashion, should theenergy collected by the antennas fall only within the pass band ofoscillator-detector 2a, both oscillator-detectors 2, 2a are again causedto transmit high frequency energy from antennas 1, la.

The use of two or more oscillator-detectors is unexpectedlyadvantageous, yielding improvement beyond that improvement which wouldnormally be expected merely by doubling equipment, as will be explained.While the following discussion of the improvement yielded is regarded asuseful in further explaining the invention, it is not the only analysiswhich might be used, and therefore is not offered in a restrictivesense.

The transponder of the present invention must have a transmitter thatcan be seen by a radar receiver tuned anywhere within a frequency bandAF. The quantity AF must be large enough to take into account allpossible variations in the operating frequency of the radar system andof the transponder. If two transmitters are used in the transponder, thequantity AF for each is halved. Now, it is to be recognized that a radiofrequency signal has a frequency spectrum whose main lobe is 2/T Hzwide, where T is the pulse width. Choosing a sufficiently narrow enoughpulse, the pulse spectrum can be spread so that it will be received byany receiver operating in the band AF. it is sufficient to observe thatoptimum results occur when H? AF, and the minimum power density is thenequal to P/2AF, where P is the pulse power. This is the power densitythat must be adequate to be seen by a radar receiver. Since a pulse ofthis nature will in general be very much narrower than the rise time ofthe receiver, the amplitude of the receiver output will be reduced byapproximately the ratio of the transponders pulse width to the radarreceiver's design pulse width. Consequently, the power density that mustbe produced at the radar receiver is also inversely proportional to thetransponder's pulse width. Providing two transmitters, each of whichcovers half the frequency range AF, permits use of an effectivelydoubled pulse width and reduces the peak transponder transmitter powerrequired by a factor of four.

In general, for a given frequency coverage and for a given cooperatingradar receiver, the peak power required of the transponder transmitteris proportional to l/N where N is the number of transponder transmittersemployed.

Operation of plural oscillator-detector devices, each with itsassociated signal processing channel, assures relatively higher poweroperation of the transponder device and thus permits more readydetection of the transponder by cooperating search radar systems. Whilea cooperating radar unit may have its operating carrier frequencycentered in the pass band of one of the plural oscillator-detectors, theuse of plural oscillator-detectors ensures that the total frequency bandreserved for use in marine radar applications is covered by thetransponder. Also assured is that a portion of the transmission from atransponder always falls in the receptivity band of all cooperatingradar receivers. Accordingly, the invention responds essentially like anuntuned transponder to any incoming signal within its composite band andtransponds over the entire marine radar band, making its response idealfor use as a distress signalling device.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than of limitation and changes within the purview ofthe appended claims may be made without departure from the true scopeand spirit of the invention in its broader aspects.

lclaim:

1. Radio transponder apparatus comprising:

oscillator-detector means,

bias current supply means for continuously biasing saidoscillator-detector means near its oscillating state, antenna means forcoupling a pulsed high frequency energy transmission into saidoscillator-detector means for causing onset of oscillations therein,

means for detecting said onset of oscillations for producing a triggerpulse thereupon,

means triggered into operation by said trigger pulse for producing asubstantially standardized pulse, and

means for coupling a version of said substantially standardized pulse tosaid oscillator-detector means for causing said oscillator-detectormeans to generate a pulsed high frequency signal for transmission out ofsaid antenna means.

2. Apparatus as described in claim 1 wherein said oscillator-detectormeans comprises negative-resistance diode oscillator-detector means.

. 3. Apparatus as described in claim 1 wherein said oscillator-detectormeans comprises semiconductor diode oscillator-means having single portmeans adapted for connection to said antenna means.

4. Apparatus as described in claim 2 having means for supplying saidbias current through said diode means.

5. Apparatus as described in claim 4 wherein said bias current supplymeans comprises bias source means coupled through impedance means tosaid diode means.

6. Apparatus as described in claim 5 wherein said means for detectingsaid onset of oscillations for producing a trigger pulse comprisescircuit means including capacitive means for sensing voltage changesacross said impedance means.

7. Apparatus as described in claim 6 wherein said circuit meanscomprises impedance means and amplifier means in series relation withsaid capacitive means.

8. Apparatus as described in claim 7 comprising diode clipper meanscoupled between said circuit means and ground for protecting saidamplifier means.

9. Apparatus as described in claim 6 wherein said means for supplying astandardized pulse comprises single shot multivibrator means having anoutput unilaterally coupled to said capacitive means.

10. Radio transponder apparatus comprising:

first and second oscillator-detector means,

bias current supply means for biasing said oscillatordetector means nearthe respective oscillating states of said oscillator-detector means,

antenna means for coupling pulsed high frequency energy into at leastone of said first and second oscillator-detector means for causing onsetof oscillations therein,

means for detecting said onset of oscillations within saidoscillator-detector means for producing a trigger pulse thereupon,

means, triggered into operation upon receiving said trigger pulse forproducing a substantially standard pulse, and

means for coupling a version of said substantially standard pulse tosaid first and second oscillator detector means for causing onset ofoscillations in both of said first and second oscillator-detector meansfor transmission of high frequency energy from antenna means.

11. Apparatus as described in claim 10 wherein:

said first and second oscillator-detector means are characterized byrespective first and second nonoverlapping operational frequency bands.

1. Radio transponder apparatus comprising: oscillator-detector means,bias current supply means for continuously biasing saidoscillator-detector means near its oscillating state, antenna means forcoupling a pulsed high frequency energy transmission into saidoscillator-detector means for causing onset of oscillations therein,means for detecting said onset of oscillations for producing a triggerpulsE thereupon, means triggered into operation by said trigger pulsefor producing a substantially standardized pulse, and means for couplinga version of said substantially standardized pulse to saidoscillator-detector means for causing said oscillator-detector means togenerate a pulsed high frequency signal for transmission out of saidantenna means.
 2. Apparatus as described in claim 1 wherein saidoscillator-detector means comprises negative-resistance diodeoscillator-detector means.
 3. Apparatus as described in claim 1 whereinsaid oscillator-detector means comprises semiconductor diodeoscillator-means having single port means adapted for connection to saidantenna means.
 4. Apparatus as described in claim 2 having means forsupplying said bias current through said diode means.
 5. Apparatus asdescribed in claim 4 wherein said bias current supply means comprisesbias source means coupled through impedance means to said diode means.6. Apparatus as described in claim 5 wherein said means for detectingsaid onset of oscillations for producing a trigger pulse comprisescircuit means including capacitive means for sensing voltage changesacross said impedance means.
 7. Apparatus as described in claim 6wherein said circuit means comprises impedance means and amplifier meansin series relation with said capacitive means.
 8. Apparatus as describedin claim 7 comprising diode clipper means coupled between said circuitmeans and ground for protecting said amplifier means.
 9. Apparatus asdescribed in claim 6 wherein said means for supplying a standardizedpulse comprises single shot multivibrator means having an outputunilaterally coupled to said capacitive means.
 10. Radio transponderapparatus comprising: first and second oscillator-detector means, biascurrent supply means for biasing said oscillator-detector means near therespective oscillating states of said oscillator-detector means, antennameans for coupling pulsed high frequency energy into at least one ofsaid first and second oscillator-detector means for causing onset ofoscillations therein, means for detecting said onset of oscillationswithin said oscillator-detector means for producing a trigger pulsethereupon, means, triggered into operation upon receiving said triggerpulse for producing a substantially standard pulse, and means forcoupling a version of said substantially standard pulse to said firstand second oscillator-detector means for causing onset of oscillationsin both of said first and second oscillator-detector means fortransmission of high frequency energy from antenna means.
 11. Apparatusas described in claim 10 wherein: said first and secondoscillator-detector means are characterized by respective first andsecond non-overlapping operational frequency bands.